Well, I've been playing this game off and on for the last year or so, my experiences:
First fort ever:
"Oh, yay, I got everything working! Ok, so I'm supposed to press this button to start digging... Oh, wait, what does this say? Crap, I can't figure out the controls." All said 10 seconds before my group were attacked by some unexpected wildlife and slaughtered.
Second fort ever:
"Ok, so I need defenses, let's channel this river over here." *Goes down a z-level* "Wait, my farms were right there. And I wasn't paying attention long enough to have failed to quarantine my only stairwell! Well, redo!"
Last fort played:
"Yay, I've been out here a year, I have 37 dwarfs, and a stable, functioning society. No one's unhappy, I have a source of shell, miners just reported a coal vein, and I just built a wall around our primary entrance. Things are good!" Then an ambush of 2 goblin squads appears, so I remove the up parts of my main entrance stairs, sealing off the fort from the surface, and locked the only remaining door. After about 10 minutes, I couldn't find the ambushers (as I had neglected to check the units list), so I unlocked the door. 10 seconds and a rush at the dead elves' clothes nearby later 34 dwarfs lay dead, and 3 miners were still as happy as can be way at the bottom, mining out our 50th bedroom.
Well, that was until they walked outside and all simultaneously went berserk. *sigh*